Reamer tool positionable in a wellbore

ABSTRACT

Described herein is a reamer tool ( 100 ) having a body ( 105 ) with bays ( 115 ) in which cutter arms ( 110 ) are mounted for deployment between a stowed position and a deployed position. A deployment mechanism is provided for deploying the cutter arms from their stowed position to their deployed position that maintains each cutter arm in a position that is substantially parallel to a longitudinal axis of the body ( 105 ) whilst in its stowed position and in its deployed position as well as during its deployment from its stowed position to its deployed position. A control module ( 300 ) is also described for controlling the deployment of the cutter arms ( 110 ). The control module ( 300 ) comprises a motor ( 310 ), a gearing mechanism ( 315 ) and a moveable element ( 320 ) that closes a port ( 385 ) in a first position and opens the port ( 385 ) in a second position. Fluid flow enters a chamber ( 340 ) behind a piston ( 170 ) through the port ( 385 ) to allow pressure to build up before actuating the piston ( 170 ) and thereby the deployment mechanism for the cutter arms ( 170 ).

FIELD OF THE INVENTION

The present invention relates to improvements in or relating to downholetools, and is more particularly, although not exclusively, concernedwith reamer tools.

BACKGROUND TO THE INVENTION

Earth formation drilling utilises a long string of drilling pipes andtools coupled together. All elements of the drilling string are rotatedtogether in order to rotate a cutting bit at the end of the drillingsting. The cutting bit creates a hole in a formation through which therest of the drilling string moves in a drilling direction. Anunder-reamer, coupled between two other elements of the drilling string,is used to widen the walls of the hole created by the drill bit. Theunder-reamer, also known as a reamer, normally has an overall diameterin its retracted position which is the same as or less than the diameterof the hole being drilled. When in its deployed position, cuttingelements are moved away from the body of the under-reamer to define adiameter which is larger than the diameter of the hole being drilled. Asthe under-reamer moves downhole rotating with the drilling string, itwidens the hole in the formation behind the drill bit. In addition, anunder-reamer may be used to open a collapsed formation on its way backup to the surface.

WO-A-2005/124094 describes one such under-reamer or reamer tool. Thereamer tool comprises a tubular body having an axial cavity and housingsarranged around its periphery to define external openings. In each ofthese openings, a cutter element is housed which comprises two cutterarms that can be moved between a retracted position where each cutterelement is fully retained within its associated housing, and an expandedposition where each cutting element extends outside its opening so thatmore material can be cut away the walls of the hole in a formationthereby enlarging its diameter. A drive mechanism is provided within thetubular body to move the cutter elements between their retracted andexpanded positions.

In the reamer tool described in WO-A-2005/124094, one cutter arm ispivotally connected to the tubular body at one end and to the othercutter arm at the other end, the other cutter arm being connected to thedrive mechanism so that both cutter arms can be retracted and expanded.The arrangement formed by the two cutter arms when deployed is a‘V’-shape where the vertex of the V is outside the opening.

Typically, such reamer tools are operated by the pressure of fluidpassing through the drill string, and in particular, through the toolsection itself. The pressure of fluid is controlled by the operation ofa pump associated with the drill string. In US-A-2010/0006339, thepressure of fluid passing through the tool is used to operate the reamerso that it is expanded or retracted in accordance therewith. Here, thereamer assembly comprises cutter elements and stabiliser pads mountedfor sliding movement on grooves. In the retracted position, the reamerassembly is housed within a recess, the reamer assembly being moved tothe expanded position by movement along the grooves so that it isoutside the recess. Fluid pressure is sensed to activate the expansionand retraction of the reamer.

US-A-2010/0096191 discloses an under-reaming and stabilisation tool inwhich a blade element is moved from a retracted position to an expandedposition by wedge elements coupled to a drive tube, the wedge elementsinteract with an inclined face of the blade element to effect theraising (expansion) and lowering (retraction) of the blade elementrelative to a guide channel. As the drive tube moves along the length ofthe tool body, the wedge elements are drawn along therewith and theyslide under the inclined face of the blade element causing radialmovement of the blade element to raise out (expand) out of its guidechannel. Movement of the drive tube in the opposite direction along thelength of the tool body withdraws the wedge elements from under theinclined face of the blade element allowing radial movement of the bladeelement to lower (retract) into its guide channel. The expansion of theblade element is limited by the actuation mechanism, that is, the drivetube and wedge elements coupled thereto.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved reamer tool in which the cutter arms or blades are maintainedparallel to the axis of the reamer tool in both its retracted anddeployed positions as well as during expansion and retraction whilstproviding a higher opening range.

It is a further object of the present invention to provide a reamer toolin which the opening can be adjusted at the surface in accordance with avalue within the opening range whilst providing a more efficient reamertool.

In accordance with a first aspect of the present invention, there isprovided a reamer tool comprising:

a substantially hollow body having a longitudinal axis and including anexternal wall having a first outer diameter;

at least one arm bay formed in a portion of the external wall around theperiphery of the body;

at least one expandable arm located in an associated arm bay and mountedfor expansion between a retracted position within the body and anexpanded position in which each expandable arm describes a second outerdiameter which is greater than the first outer diameter; and

at least one expansion mechanism for expanding an associated expandablearm between the retracted and expanded positions;

characterised in that each expansion mechanism comprises two elongatelinks pivotally connected to the associated expandable arm at one endposition and to its associated arm bay at another end position, eachexpandable arm being pivotally mounted at the two end positions withrespect to its associated arm bay so that each expandable arm ismaintained substantially parallel to the longitudinal axis of the bodyin both the retracted and expanded positions and during its expansionand retraction between the retracted and expanded positions.

By having links connecting each expandable arm to its associated armbay, the expandable arm can be maintained substantially parallel to thelongitudinal axis of the reamer tool thereby providing an opening rangewhich is greater than that possible with expansion mechanisms comprisingwedge elements or the like.

In the case where the downhole tool comprises a reamer tool, theadvantage of maintaining the expandable arm parallel to the longitudinalaxis of the body is that the attack point for each cutting blade isreliable, the attack point being the point at which a leading cuttingelement engages with the material or formation to be cut.

Naturally, an actuation mechanism is also provided for activating theexpansion mechanism, each expandable arm being pivotally connected atanother end position to the actuation mechanism.

Advantageously, the expansion mechanism further comprises a thirdelongate link pivotally connected to each expandable arm and to theactuation mechanism.

In this way, the actuation mechanism directly moves the expandable armand the other elongate links serve to maintain the substantialparallelism with the longitudinal axis. In a preferred embodiment, theactuation mechanism comprises a piston.

The downhole tool may further comprise at least one return member fordeactivating each deployment mechanism. In one embodiment, each returnmember comprises a spring biased against the action of the actuationmechanism.

A shoulder block may be provided which is locatable in each arm bay tolimit the expansion of the expandable arm. By selecting a suitably sizedshoulder block, the expansion of the expandable arm can be determined toprovide a desired outer diameter for engagement with a formation.

In a preferred embodiment, the second outer diameter may be up to 1.3times the first outer diameter. For example, if the outer diameter ofthe downhole tool is 100 cm, the expandable arms may be expanded todescribe an outer diameter of up to 130 cm.

Preferably, the downhole tool comprises a reamer tool and eachexpandable arm comprises a cutter arm.

In accordance with another aspect of the present invention, there isprovided an expandable cutter arm for a downhole tool, the expandablecutter arm comprising at least a front cutting blade and a back cuttingblade, each cutting blade comprising a plurality of cutting elements,one cutting element on each of the front cutting blade and the backcutting blade providing an attack point for the associated cuttingblade.

Such an expandable cutter arm may further comprise a first side and asecond side located either side of a plane, each side being spaced atrespective predetermined distances from a plane so that the attack pointfor the front blade and the attack point for the back blade areequi-spaced from the plane.

By having the attack point for each cutter arm equi-spaced from theplane, efficiency of the reamer tool is improved. In addition, a moreflexible reamer tool is provided in which a range of opening sizes canbe accommodated.

The predetermined distance for the first side may be different to thepredetermined distance for the second side.

In one embodiment, the cutting elements may comprise polycrystallinediamond cutting elements.

In accordance with a further aspect of the present invention, there isprovided a reamer tool having at least one expandable cutter arm asdescribed above.

In accordance with another aspect of the present invention, there isprovided a reamer tool having a longitudinal axis, the reamer toolcomprising at least one expandable cutter arm having a plurality ofcutting elements arranged to form at least a front cutting blade and aback cutting blade, one of the cutting elements on the front cuttingblade and one of the cutting elements on the back cutting bladeproviding respective attack points for their associated cutting blades,characterised in that the attack point for the front cutting blade andthe attack point for the back cutting blade are equi-spaced from a planeextending through the longitudinal axis.

The reamer tool preferably further comprises at least one expansionmechanism for expanding an associated expandable cutter arm between aretracted position and an expanded position, and an actuation mechanismfor activating each expansion mechanism.

In a preferred embodiment, each expansion mechanism comprises at leasttwo elongate links pivotally connected to the associated expandablecutter arm at one end position and to its associated arm bay at anotherend position, each expandable cutter arm being pivotally mounted at thetwo end positions with respect to its associated arm bas so that eachexpandable cutter arm is maintained substantially parallel to thelongitudinal axis in both the retracted and expanded positions, and,during expansion and retraction between the retracted and expandedpositions.

The expansion mechanism advantageously further comprises a thirdelongate link pivotally connected to each expandable cutter arm and tothe actuation mechanism, each expandable cutter arm being pivotallyconnected at another end position to the actuation mechanism.

The actuation mechanism preferably comprises a piston. The reamer toolmay further comprise at least one return member for deactivating eachexpansion mechanism.

A shoulder block may be provided which is locatable in each arm bay tolimit the expansion of the expandable cutter arm. The cutter arm mayhave an opening range up to 1.3 times the outer diameter of the reamertool, the shoulder block limiting the opening in accordance with itsize.

In accordance with another aspect of the present invention, there isprovided a control module for a downhole tool, the downhole toolincluding a substantially hollow body having a longitudinal axis, atleast one arm bay formed around the periphery of the substantiallyhollow body, at least one expandable arm located in an associated armbay and mounted for expansion between a retracted position within thesubstantially hollow body and an expanded position in which theexpandable arm describes a second outer diameter which is greater thanthe first outer diameter, at least one expansion mechanism for expandingan associated expandable arm between the retracted and expandedpositions, and a piston for operating each expandable arm, the controlmodule comprising:

an element mounted within the body which is moveable between a firstposition and second position;

a motor controlling the movement of the element; and

a gearing mechanism associated with the motor for transferring drivefrom the motor to the element;

characterised in that the control module further comprises a chamber anda port, the chamber being associated with the piston and the port havingan open position and a closed position, the open and closed positionbeing determined by the second and first positions respectively of theelement;

and in that the port, in its open position, allows fluid to flow intothe chamber and to increase the pressure therein for operation of thepiston to expand each expandable arm.

In a preferred embodiment, the motor and gearing mechanism are mountedbetween the element and the external wall of the body. A power source ispreferably located within the body of the reamer tool. This has theadvantage of protecting the control module, that is, the motor, gearingmechanism and power source from the environment in which the reamer tooloperates.

In one embodiment, the power source comprises a battery. In anotherembodiment, the power source comprises a turbine arranged to generatepower for the motor.

The control module may further comprise at least one positional sensorfor sensing the position of the element within the body. In addition, atleast one pressure sensor may also be provided for sensing the pressurewithin the chamber.

In addition, at least one sensor may be provided for sensing at least achange in pressure in fluid flowing through the downhole tool, eachsensor providing a control signal for the motor. Moreover, at least onesensor may be provided for sensing a change in rotational speed of thedownhole tool, each sensor providing a control signal for the motor.

Additionally, a communications system may be provided through whichcontrol signals are provided for the motor. In one embodiment, thecommunications system includes a wired link over which control signalsare transmitted.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference will nowbe made, by way of example only, to the accompanying drawings in which:

FIG. 1 illustrates a schematic sectioned view of a reamer tool inaccordance with the present invention, the reamer tool being shown in aretracted position;

FIG. 2 is similar to FIG. 1 but illustrates the reamer tool in anexpanded position;

FIG. 3 illustrates cutters mounted on an arm of the reamer tool shown inFIGS. 1 and 2;

FIG. 4 illustrates a sectioned view of a control system for the reamertool shown FIGS. 1 and 2 with the reamer tool in the stowed position;

FIG. 5 is similar to FIG. 4 but illustrates the control system with thereamer tool in the expanded position.

DESCRIPTION OF THE INVENTION

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto. The drawings described are only schematic and arenon-limiting. In the drawings, the size of some of the elements may beexaggerated and not drawn on scale for illustrative purposes.

It will be understood that the terms “vertical” and “horizontal” areused herein refer to particular orientations of the Figures and theseterms are not limitations to the specific embodiments described herein.In addition, the terms “top” and “bottom” are used to refer to parts ofa drill string that face towards the surface, or top of the drillstring, and away from the surface, or bottom of the drill string,respectively.

The present invention relates to an improved reamer tool and a controlsystem for operating such a reamer tool or other downhole tool. Thereamer tool is described below with reference to FIGS. 1 to 3 and thecontrol system is described with reference to FIGS. 4 and 5.

Although the present invention is described below with respect to areamer tool having cutter arms, it is equally applicable to a downholetool that may also be used for stabilisation. In this case, the cutterarms are replaced by stabilising pad arms which, when expanded, contactthe walls of a formation to stabilise the drill string of which the toolforms a part. In addition, although the control system is described withreference to use with a reamer tool, it is not limited to use with areamer tool and can be used with any other downhole tool.

Reamer tools, as well as other downhole tools, are operated, that is,expanded and retracted by changes in the pressure of fluid flowingthrough the associated drill string. The fluid flow is controlled by apump associated with the drill string. Changes in fluid pressure aredetected by sensors located at appropriate positions in the drillstring.

Referring initially to FIGS. 1 and 2, a longitudinal sectioned view ofreamer tool 100 is shown. The reamer tool 100 comprises a reamer body105 having three cutter arms 110 mounted within respective housings orarm bays 115 formed in the reamer body 105. The three cutter arms 110are equi-spaced around the periphery of the reamer body 105 but only onesuch cutter arm is shown in FIGS. 1 and 2.

Each cutter arm 110 comprises a cutting element or cutting blade 120which is pivotally mounted on each of three elongate links 125, 130, 135at respective pivot points 140, 145, 150 as shown. Two of the elongatelinks 125, 130 are also pivotally attached to the housing or arm bay 115at respective pivot points 155, 160. The third elongate link 135 is alsopivotally mounted, by means of a pivot point 165, on a piston 170.

The piston 170 comprises an actuation mechanism and is operated to movefrom a first position as shown in FIG. 1 to a second position as shownin FIG. 2 to expand the cutter arms 110, and more particularly, thecutting elements or cutting blades 120, from a retracted position to anexpanded position where the cutting elements or cutting blades 120extend outside the reamer body 105 and define an outer diameter which isup to 1.3 times that of the normal outer diameter of the reamer body105.

It will be appreciated that, in other embodiments of the reamer tool 100in accordance with the present invention, the outer diameter defined bythe three cutter arms 110 and their cutting elements or cutting blades120 may have other ratios compared to the outer diameter of the reamerbody 105 as required, and, is therefore not limited to up to 1.3 timesthe outer diameter of the reamer body 105. The outer diameter is limitedby a shoulder block 175 and the size of the shoulder block 175 is chosenat the surface before introduction of the drill string of which thereamer tool 100 forms a part into a wellbore in a formation inaccordance with the outer diameter of the reamer tool 100 required tofrom the wellbore in the formation.

It will be appreciated that shoulder blocks of different sizes can beprovided with the reamer tool 100 and an appropriately sized shoulderblock is chosen to limit the expansion of the cutter arms 110 to controlthe outer diameter defined by the expanded cutter arms 110 and cuttingelements or cutting blades 120 within an opening range from the sameouter diameter of the reamer body 105 to 1.3 times that outer diameter.

In the deployment of the cutter arms 110 from inside their respectivehousings or arm bays 115 formed in the reamer body 105, the cuttingstructure (not shown) of each cutter arm 110 always remains parallel toa longitudinal axis 180 of the reamer body 105. The pivot points 140,145, 150, 155, 160, 165 formed on respective ones of the links 125, 130,135, as described above, effectively provide pivoting axes about whichrotation can occur to expand and retract the cutter arms 110 and cuttingelements or cutting blades 120 out of and into their respective housingsor arm bays 115. However, pivot points 140, 145 provided on respectivelinks 125, 130 ensure that the cutter arms 110 remain parallel to thereamer body 105 as they are expanded, used for cutting and retractedinto their respective housings or arm bays 115. Pivot point 150 providedon elongate link 135 is used to expand and retract the associated cutterarm 110 in accordance with the movement of the piston 170 or otheractuation mechanism as will be described in more detail below.

By using an expansion mechanism which utilises elongate links pivotallyconnected to both the cutter arm 110 and the housing or arm bay 115 aswell as to the piston 170 or other actuation mechanism, the effectiveouter diameter of the cutter arm 110 and cutting element or cuttingblade 120 can extend up to 1.3 times the outer diameter of the reamerbody 105. In addition, the amount of expansion can easily be limited bya suitable shoulder block 175.

The force for expanding the cutter arms 110 is provided by pressureapplied to the piston 170, and, the force for retracting the cutter armsis provided by a spring 185 (described below with reference to FIGS. 4and 5). The applied pressure is provided by fluid flow through thereamer body 105 as will be described in more detail below.

As shown in FIGS. 1 and 2, the reamer body 105 is substantially tubularwith a hollow central portion 190 which defines a fluid flow path. Thepiston 170 is mounted within the reamer body 105 and is operated byfluid flowing therethrough as will described in more detail withreference to FIGS. 4 and 5 below.

In the embodiment of the reamer tool 100 described above, it isessential to ensure that the cutting elements, for example,polycrystalline diamond cutters known as PDC cutters, functionadequately during the expansion stages to make contact with theformation in which the reamer tool is to be used. This is described inmore detail with respect to FIG. 3.

In FIG. 3, a portion 200 of a cutter arm 110 of the reamer tool 100shown in FIGS. 1 and 2 is shown in more detail. The positioning of thecutting elements with respect to the cutter arm 110 is shown. Theportion 200 shows a single cutter arm 110 (FIG. 1) having two cuttingblades 205, 210, a front cutting blade 205 and a back cutting blade 210.[The terms “front” and “back” refer to the order in which the cuttingblades make contact with the walls of a wellbore formed in a formationand is determined by the direction of rotation of the drill string (notshown) of which the reamer tool 100 (FIG. 1) forms a part.]

In the embodiment shown in FIG. 3, five cutting elements 215, 220, 225,230, 235 are visible on front cutting blade 205, and six cuttingelements 240, 245, 250, 255, 260, 265 are visible on back cutting blade210. Cutting element 215 on front cutting blade 205 and cutting element240 on back cutting blade 210 have respective attack points 270, 275which are equi-spaced from a plane 280 that is coincident with thelongitudinal axis 180 of the reamer body 105 (FIG. 1). This means thatthe distance from side 285 of front cutting blade 205 to the plane 280is shorter than the distance from side 290 of back cutting blade 210 toplane 280.

In the embodiment shown in FIG. 3, the cutting elements 215, 220, 225,230, 235, 240, 245, 250, 255, 260, 265 comprise PDC elements as shown.Although eleven PDC elements are visible, the number of PDC elementspresent on each blade 205, 210 is determined in accordance with thedimensions of the PDC element and the dimension of the reamer toolitself. However, it will be appreciated that other types of cuttingelements may also be used, for example, impregnated cutting elements.

By having the attack points 270, 275 equi-spaced from the plane 280,attack points 270, 275 will contact the formation for any opening sizein the opening range. If the attack points 270, 275 are not equi-distantfrom the plane 280, the cutter arms will only have one possible openingsize to ensure that both the front and back cutting blades make contactwith the formation.

The front and back blades 205, 210 as described above have differentnumbers of cutting elements 215, 220, 225, 230, 235, 240, 245, 250, 255,260, 265 which are not aligned with one another so that the attackpoints 270, 275 of cutting elements 215, 240 are at different heightswith respect to the reamer body 105.

The effective outer diameter of the reamer tool 100, that is, theopening size is determined by the positions of attack points 270, 275.

Referring now to FIGS. 4 and 5, a schematic longitudinal sectioned viewof the reamer tool 100 is shown. Components that have previously beendescribed with reference to FIGS. 1 and 2 have the same referencenumerals.

The reamer tool 100 comprises the reamer body 105 having cutter arms 110mounted within respective housings or arm bays 115 formed in the reamerbody 105 as described above. The links and the pivot points that operatethe cutter arms 110 as described above are not shown for clarity. Thespring 185 that is used to return the expanded cutter arms to theirretracted position is shown schematically as a block.

As described above, the force for expanding the cutter arms 110 isprovided by pressure applied to the piston 170 due to fluid flow throughthe reamer tool 100, and, the force for retracting the cutter arms isprovided by the spring 185. During expansion of the cutter arms, thepressure exerted on the piston 170 creates a force which is greater thanthe force provided by the spring 185. Once the pressure exerted on thepiston 170 falls sufficiently so that the force exerted becomes lessthan the force provided by the spring 185, the spring 185 causes thecutter arms 110 to be retracted into their respective housings or armbays 115. This is described in more detail below.

A control system 300 for deploying the cutter arms 110 is providedwithin the reamer body 105 and comprises an electric motor 310, agearing system 315 and a moveable sleeve 320, the electric motor 310 andgearing system 315 being housed between the sleeve 320 and an externalwall 325 of the reamer body 105. The electric motor 310 rotates at afirst predetermined speed and the gearing system 315 reduces that firstpredetermined speed to a second lower predetermined speed which is usedfor operating the moveable sleeve 320. In one embodiment, a ball screw(not shown) may be used to transfer the rotational output from thegearing system 315 to a linear movement which is used to move the sleeve320 to open and close port 385 as will be described in more detailbelow. However, it will be appreciated that other arrangements may beused for transferring rotary motion from the gearing system 315 tolinear motion of the moveable sleeve 320, for example, a pinion or wormgear forming part of the gearing system 315 may engage with a rackelement provided on the moveable sleeve 320.

The electric motor 310 may be powered by a battery (not shown) or from aturbine provided in the drill string (also not shown), the turbinegenerating a current from the fluid flow therethrough. Although agearing system 315 is described, it will be appreciated that drive fromthe motor may be converted into linear movement by any suitable meansfor converting the output of the motor into linear movement.

The housing or arm bay 115 for each cutter arm 110 is defined by a wall330 of the hollow central portion 190 and a portion 335 of the externalwall 325 of the reamer body 105. The piston 170 is defined by a chamber340 adjacent the cutter arm 110, the chamber 340 being defined by thewall 330 of the central portion 190, external wall 325 of the reamerbody 105, sleeve 320, first cylindrical portion 345, second cylindricalportion 350 and end wall 355 as shown. End wall 355 also forms barrierbetween the electric motor 310 and gearing system 315 of the controlsystem 300.

Annular seals 360, 365 are provided between the first cylindricalportion 345 and respective ones of wall 330 and sleeve 320. Additionalannular seals 370, 375 are provided between sleeve 320 and secondcylindrical portion 350 and with wall 380 of hollow central portion 190.Seal 360 can be mounted on either the first cylindrical portion 345 orthe wall 330 as the first cylindrical portion 345 does not move relativeto the wall 330.

The first and second cylindrical portions 345, 350 define the port 385which is sealed by the moveable sleeve 320 when in a first position, asshown in FIG. 4, so that fluid flows through the hollow central portion190 as indicated by arrow 390. When the sleeve 320 is in a secondposition, as shown in FIG. 5, the port 385 is open and fluid can flowinto chamber 340 as shown by arrow 395.

An additional seal 400 is also provided between the piston 170 and theexternal wall 325 of the reamer body 105 as shown to prevent ingress ofdrilling fluid as the piston 170 moves from the position shown in FIG. 4to the position shown in FIG. 5.

Operation of the electric motor 310 effectively moves the sleeve 320 inthe same direction as arrow 390 to open the port 385 and in the oppositedirection to close the port 385, drive from the electric motor 310 beingtransmitted to the sleeve 320 via the gearing system 315. A controlsignal for the electric motor 310 is provided by way of an increasedfluid flow rate through the hollow central portion 190 and/or speed ofrotation of the drill string (not shown). At least one suitable sensor(not shown) is provided to sense the change in pressure and/orrotational speed and to provide a control signal for the electric motor310, for example, a pressure sensor for sensing changes in pressure andan accelerometer for sensing the change in rotational speed. However,other sensors may also be used for sensing the change in rotationalspeed.

It will be appreciated that the electric motor 310 may be abi-directional motor that operates in two directions to effect openingand closing of the port 385. As an alternative to the electric motor310, a solenoid may be used to effect opening and closing of the port385.

Naturally, the electric motor 310 and gearing system 315 are sealedwithin a region 410 defined by the sleeve 320 and an external wall 325so that it is protected from the drilling environment, that is, the mud,rock etc., that finds its way into the hollow central region 190. In apreferred embodiment, the region 410 is filled with oil to prevent theingress debris from the drilling environment.

Before the cutter arms 110 are expanded, they are housed in theirrespective housings or arm bays 115 as described above. Fluid flow isthrough the hollow central portion 190 as indicated by arrow 390 (FIG.4). When a control signal is sent to the electric motor 310, by way of achange in pressure of the fluid flowing through the hollow centralportion 190 and/or a change in the rotational speed of the drill stringas described above, the electric motor 310 operates the moveable sleeve320 to move it in the same direction as the fluid flow as indicated byarrow 390 to open port 385 (FIG. 5).

When the port 385 is opened, fluid flows into the chamber 340 andpressure builds up therein. When the pressure in the chamber 340 reachesa value where the force exerted by the piston 170 is greater than theforce exerted by the spring 185, the piston 170 is pushed from theposition shown in FIG. 4 towards the arm bays 115 to expand the cutterarms 110 as shown in FIG. 5. Movement of the piston 170 towards the armbays 115 causes each cutter arm 110 to pivot about pivot point 150 onlink 135, as well as pivot points 140, 145 on links 125, 130, so that itis expanded from the within its associated arm bay 115 as shown in FIGS.1 and 4, to the position as shown in FIGS. 2 and 5. Fluid built up inthe chamber 340 flows out of nozzles 415 associated with the cutter arms110 maintaining the position of the piston 170 as shown in FIGS. 2 and5, and hence the expansion of the cutter arms 110, until the port 385 isclosed by the sleeve 320 by the operation of the motor 310 and gearingmechanism 315.

On receipt of a further control signal, that is, another change inpressure of the fluid flow and/or a change in rotational speed of thedrill string, the motor 310 is activated once again to move the moveablesleeve 320 from the position shown in FIG. 5 back to the position shownin FIG. 4, thereby closing the port 385 so that no more fluid flows intothe chamber 340 as indicated by arrow 395. Fluid flows out of nozzles415 until the pressure in the chamber 340 is reduced so that the forceof the spring 185 causes the cutter arms 110 to be returned to theirassociated housing or arm bay 115 to be returned to the position shownin FIGS. 1 and 4. In addition, the piston 170 is pushed back but theforce exerted by the spring 185 to its initial position as shown inFIGS. 1 and 4.

Alternatively, instead of operating the motor 310, the cutter arms 110may be retracted by turning the pump off that is associated with thedrill string so that fluid flow is switched off through the drillstring, and the pressure in the chamber 340 falls as no further fluidflows through the port 385 and into the chamber 340. Once the pressurein the chamber 340 falls to a value where the force exerted by thespring 185 exceeds that of provided by the pressure in the chamber 340,the piston 170 is moved back to the position shown in FIGS. 1 and 4 andthe cutting arms 110 retracted whilst still parallel to the longitudinalaxis 180 due to their pivoting about points 140, 145, 150; pivoting ofthe links 125, 130 about points 155, 160 in the respective housing orarm bay 115; and pivoting about pivot point 165 due to movement of thepiston 170 as it moves from the position shown in FIG. 5 back to theposition shown in FIG. 4.

As mentioned above, the control system 300 includes a power supply (notshown), but it may also include other electronic equipment, for example,pressure sensors for sensing the pressure in the chamber 340,accelerometers for measuring the speed of movement of the sleeve 320 andpiston 170 and the rotational speed of the drill string, as well as thespeed of the cutter arm 110 during its expansion and retraction phases.In addition, a communication device (not shown) may be provided throughwhich control signals can be provided for the electric motor in the casewhere the control signals are not supplied by changes in pressure of thefluid flow or rotational speed of the drill string as described above.

The power supply may be provided by one or more batteries or via a wiredlink from the surface. Additionally, the wired link may form part of thecommunication device through which the control signals may betransmitted to the electric motor.

It will be appreciated that the cutter arm expansion mechanism can beused with other tools, for example, downhole stabilisers, and the cutterarms can be expanded using other expansion mechanisms.

Although a specific embodiment of the present invention is described, itwill be appreciated that this embodiment is not limiting and otherembodiments may fall within the scope of the invention as defined by theappended claims.

1. A reamer tool positionable downhole in a wellbore, said toolcomprising: a substantially hollow body having a longitudinal axis andincluding an external wall having a first outer diameter; at least onearm bay formed in a portion of the external wall around the periphery ofthe body; at least one expandable arm located in an associated arm bayand mounted for expansion between a retracted position within the bodyand an expanded position in which each expandable arm describes a secondouter diameter which is greater than the first outer diameter; and atleast one expansion mechanism for expanding an associated expandable armbetween the retracted and expanded positions; wherein each expansionmechanism comprises two elongate links pivotally connected to theassociated expandable arm at one end position and to its associated armbay at another end position, each expandable arm being pivotally mountedat the two end positions with respect to its associated arm bay so thateach expandable arm is maintained substantially parallel to thelongitudinal axis in both the retracted and expanded positions, and,during its expansion and retraction between the retracted and expandedpositions.
 2. The reamer tool of claim 1, further comprising anactuation mechanism for activating the expansion mechanism, eachexpandable arm being pivotally connected at another end position to theactuation mechanism.
 3. (canceled)
 4. (canceled)
 5. The reamer tool ofclaim 1, further comprising at least one return member for deactivatingeach expansion mechanism.
 6. The reamer tool of claim 5, wherein eachreturn member comprises a spring biased against the action of theactuating mechanism.
 7. The reamer tool of claim 1, further comprising ashoulder block locatable in each arm bay to limit the expansion of theexpandable arm.
 8. The reamer tool of claim 1, wherein the second outerdiameter is up to 1.3 times the first outer diameter.
 9. The reamer toolof claim 1, wherein each expandable arm comprises a cutter arm.
 10. Anexpandable cutter arm for a reamer tool postionable downhole in awellbore, the expandable cutter arm comprising at least a front cuttingblade and a back cutting blade, each cutting blade comprising aplurality of cutting elements, one cutting element on each of the frontcutting blade and the back cutting blade providing an attack point forthe associated cutting blade.
 11. The expandable cutter arm of claim 10,further comprising a first side and a second side located either side ofa plane, each side being spaced at respective predetermined distancesfrom the plane so that the attack point for the front blade and theattack point for the back blade are equi-spaced from the plane.
 12. Theexpandable cutter arm of claim 11, wherein the predetermined distancefor the first side is different to the predetermined distance for thesecond side.
 13. The expandable cutter arm of claim 10, wherein thecutting elements comprise polycrystalline diamond cutting element.
 14. Areamer tool postionable downhole in a wellbore, said tool having atleast one expandable cutter arm comprising: at least a front cuttingblade and a back cutting blade, each cutting blade comprising aplurality of cutting elements, one cutting element on each of the frontcutting blade and the back cutting blade providing an attack point forthe associated cutting blade; and a first side and a second side locatedeither side of a plane, each side being spaced at respectivepredetermined distances from the plane so that the attack point for thefront blade and the attack point for the back blade are equi-spaced fromthe plane, and wherein the predetermined distance for the first side isdifferent to the predetermined distance for the second side.
 15. Areamer tool positionable downhole in a wellbore, said tool having alongitudinal axis, the reamer tool comprising at least one expandablecutter arm, each expandable cutter arm having a plurality of cuttingelements arranged to form at least a front cutting blade and a backcutting blade, one of the cutting elements on the front cutting bladeand one of the cutting elements on the back cutting blade providingrespective attack points for their associated cutting blades, whereinthe attack point for the front cutting blade and the attack point forthe back cutting blade are equi-spaced from a plane extending throughthe longitudinal axis.
 16. The reamer tool of claim 15, furthercomprising at least one expansion mechanism for expanding an associatedexpandable cutter arm between a retracted position and an expandedposition, and an actuation mechanism for activating each expansionmechanism.
 17. The reamer tool of claim 16, wherein each expansionmechanism comprises at least two elongate links pivotally connected tothe associated expandable cutter arm at one end position and to itsassociated arm bay at another end position, each expandable cutter armare being pivotally mounted at the two positions with respect to itsassociated arm bay so that each expandable cutter arm is maintainedsubstantially parallel to the longitudinal axis in both the retractedand expanded positions, and, during expansion and retraction between theretracted and expanded positions.
 18. The reamer tool of claim 17,wherein the expansion mechanism further comprises a third elongate linkpivotally connected to each expandable cutter arm and to the actuationmechanism, each expandable cutter arm being pivotally connected atanother end position to the actuation mechanism.
 19. The reamer tool ofclaim 16, wherein the actuation mechanism comprises a piston.
 20. Thereamer tool of claim 16, further comprising at least one return memberfor deactivating each expansion mechanism.
 21. The reamer tool of claim15, further comprising a shoulder block locatable in each arm bay tolimit the expansion of the expandable cutter arm.
 22. The reamer tool ofclaim 21, wherein the expandable cutter arm has an opening range up to1.3 times an outer diameter of the reamer tool, the shoulder blocklimiting the opening in accordance with its size. 23-33. (canceled)